Solid electrolyte and lithium battery including the same

ABSTRACT

A solid electrolyte for an all-solid secondary battery, the solid electrolyte including: Li, S, P, an M1 element, and an M2 element, wherein the M1 element is at least one element selected from Na, K, Rb, Sc, Fr, and the M2 element is at least one element selected from F, Cl, Br, I, molar amounts of lithium and the M1 element satisfy 0&lt;M1/(Li+M1)≤0.07, and the solid electrolyte has peaks at positions of 15.42°±0.50° 2θ, 17.87° degrees±0.50° degrees 2θ, 25.48° degrees±0.50° degrees 2θ, 30.01° degrees±0.50° 2θ, and 31.38°±0.50° 2θ when analyzed by X-ray diffraction using CuKα radiation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to Korean PatentApplication No. 10-2016-0070973, filed on Jun. 8, 2016, in the KoreanIntellectual Property Office, and Japanese Patent Application No.2015-255090, filed on Dec. 25, 2015, in the Japanese Patent Office, andall the benefits accruing therefrom under 35 U.S.C. § 119, the contentsof which are incorporated herein in their entirety by reference.

BACKGROUND 1. Field

The present disclosure relates to a solid electrolyte, a lithium batteryincluding the solid electrolyte, and a method of preparing the solidelectrolyte.

2. Description of the Related Art

An all-solidified lithium battery including a solid electrolyte as anelectrolyte is considered as having excellent safety since a flammableorganic solvent is not used in the battery. In addition, the battery hasbeen examined as a promising lithium ion battery. As a solid electrolytematerial used in the solid electrolyte, a sulfide-based solidelectrolyte has been known, for example in International PatentPublication Nos. WO 2009/047254, International Patent Publication No. WO2015/011937, International Patent Publication No. WO 2015/012042,Japanese Patent No. 2011-96630, Japanese Patent No. 2013-201110,Japanese Patent No. 2015-72783, and Japanese Patent No. 2013-37897.

Nonetheless, there remains a need for an improved solid electrolyte thatis stable with respect to lithium and has suitable ion conductivity.

SUMMARY

Provided is a solid electrolyte that is stable with respect to lithiummetal and, at the same time, has a desirable ion conductivity.Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the presented embodiments.

According to an aspect of an embodiment, a solid electrolyte for anall-solid secondary battery includes Li, S, P, an M1 element, and an M2element, wherein the M1 element is at least one element selected fromNa, K, Rb, Cs, Fr, and a combination thereof, the M2 element is at leastone element selected from F, Cl, Br, I, and a combination thereof, molaramounts of Li and the M1 element satisfy 0<M1/(Li+M1)≤0.07, and thesolid electrolyte has peaks at positions of 15.42°±0.50° 2θ,17.87°±0.50° 2θ, 25.48°±0.50° 2θ, 30.01°±0.50° 2θ, and 31.38°±0.50° 2θwhen analyzed by X-ray diffraction using CuKα radiation.

According to an aspect of another embodiment, a secondary batteryincludes a cathode including a cathode active material; an anodeincluding an anode active material; and a solid electrolyte layerincluding the solid electrolyte.

Also disclosed is a method of preparing the solid electrolyte, themethod including: mechanically milling a mixture comprising Li₂S, P₂S₅,and M1₂S and LiM2, or M1M2 to obtain a glass; and heat-treating theglass at a glass transition temperature or greater to convert the glassinto and ion conductive glass ceramic and obtain the solid electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of an all-solid secondary batteryaccording to an exemplary embodiment;

FIG. 2 is a graph of intensity (arbitrary units) versus diffractionangle (degrees 2-theta (2θ)) showing the results of an X-ray diffractionmeasurements using CuKα radiation performed on solid electrolytesprepared in Examples 1 to 4 and Comparative Examples 1 to 4 of thepresent disclosure;

FIG. 3 is a graph of voltage (V) versus specific capacity (milliamperehours per gram, mAh/g) showing rate characteristics at 0.05 C ofbatteries prepared in Example 5 and Comparative Example 5;

FIG. 4 is a graph of voltage (V) versus specific capacity (mAh/g)showing rate characteristics at 0.5 C of the batteries prepared inExample 5 and Comparative Example 5;

FIG. 5 is a graph of voltage (V) versus specific capacity (mAh/g)showing rate characteristics at 1 C of the batteries prepared in Example5 and Comparative Example 5;

FIG. 6 is a graph of imaginary resistance (Z″, ohms·cm²) versus realresistance (Z′, ohms·cm²) showing internal resistance characteristics ofthe batteries prepared in Example 5 and Comparative Example 5;

FIG. 7A is a graph of current (amperes, A) versus potential (V), showingthe results of cyclic voltammetry (CV) analysis of Example 5;

FIG. 7B is a graph of current (A) versus potential (V), showing theresults of cyclic voltammetry (CV) analysis of Comparative Example 5.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, the presentembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein. Accordingly, theembodiments are merely described below, by referring to the figures, toexplain aspects. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. Expressionssuch as “at least one of,” when preceding a list of elements, modify theentire list of elements and do not modify the individual elements of thelist.

If not defined otherwise, all terms (including technical and scientificterms) in the specification may be defined as commonly understood by oneskilled in the art. The terms defined in a generally-used dictionary maynot be interpreted ideally or exaggeratedly unless clearly defined. Inaddition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising” will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements.

Further, the singular includes the plural unless mentioned otherwise.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Similar reference numerals designatesimilar elements throughout the specification.

It will be understood that when an element such as a layer, film,region, or substrate is referred to as being “on” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer, orsection. Thus, “a first element,” “component,” “region,” “layer,” or“section” discussed below could be termed a second element, component,region, layer, or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother element as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in one of the figures is turned over, elementsdescribed as being on the “lower” side of other elements would then beoriented on “upper” sides of the other elements. The exemplary term“lower,” can therefore, encompasses both an orientation of “lower” and“upper,” depending on the particular orientation of the figure.Similarly, if the device in one of the figures is turned over, elementsdescribed as “below” or “beneath” other elements would then be oriented“above” the other elements. The exemplary terms “below” or “beneath”can, therefore, encompass both an orientation of above and below.

“About” or “approximately” as used herein is inclusive of the statedvalue and means within an acceptable range of deviation for theparticular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system). For example, “about” can mean within one or morestandard deviations, or within ±30%, 20%, 10%, or 5% of the statedvalue.

Hereinafter, one or more embodiments of the present invention will bedescribed by referring to drawings. The descriptions of the embodimentsare fundamentally provided herein as examples but are not intended tolimit the equivalent or the use thereof.

A C rate means a current which will discharge a battery in one hour,e.g., a C rate for a battery having a discharge capacity of 1.6ampere-hours would be 1.6 amperes.

As used herein, the term “alkali metal” denotes an element that belongsto Group 1 in the Periodic Table.

As used herein, the term “alkaline earth metal” denotes an element thatbelongs to Group 2 in the Periodic Table.

As used herein, the term “rare earth element” denotes an element thatbelongs to the lanthanide series, and also includes scandium andyttrium.

As used herein, the term “Group XIII element” denotes an element thatbelongs to Group 13 in the Periodic Table.

As used herein, the term “Group XIV element” denotes an element thatbelongs to Group 14 in the Periodic Table.

Li-sulfide-based solid electrolytes may be used to resolve variousproblems, but ion conductivity needs to be increased. Until present, asolid electrolyte including Ge is expected to have the highest ionconductivity (10⁻² Siemens per centimeter (S/cm)), but the solidelectrolyte is unstable as the solid electrolyte is decomposed bylithium metal. Although many solid electrolytes have an ion conductivityof 10⁻⁴ S/cm and thus are stable with respect to lithium metal, asulfide-based solid electrolyte of an Argyrodite structure,Li_(6-x)PS₅Cl_(x), has an ion conductivity of 10⁻³ S/cm.

The present inventors have performed various examinations to find amaterial that is stable with respect to lithium metal and has a high ionconductivity by modifying the Argyrodite structure. As a result, thepresent inventors have discovered that ion conductivity increases byreplacing some Li with another alkali metal.

According to an embodiment, a solid electrolyte includes Li, S, P, an M1element, and an M2 element.

In an embodiment, in the solid electrolyte the M1 element may be atleast one element selected from alkali metals. In another embodiment,the M1 element may be at least one element selected from Na, K, Rb, Cs,and Fr. Also, in another embodiment, the M1 element may be at least oneelement selected from Na and K. In another embodiment, the M1 elementmay be Na.

In an embodiment, in the solid electrolyte, the M2 element may be atleast one element selected from Cl, Br, I, and F. In another embodiment,the M2 element may be Cl.

In an embodiment, in the solid electrolyte, a molar ratio of the Lielement and the M1 element may satisfy: 0<M1/(Li+M1)≤0.07.

In an embodiment, the solid electrolyte may have peaks at positions of15.42°±0.50° 2θ, 17.87°±0.50° 2θ, 25.48°±0.50° 2θ, 30.01°±0.50° 2θ, and31.38°±0.50° 2θwhen analyzed by X-ray diffraction using CuKα radiation.In an embodiment, since the solid electrolyte has an Argyroditestructure, the solid electrolyte may have X-ray diffraction peaks.

In an embodiment, in the solid electrolyte, when some Li is replacedwith another alkali metal (e.g., at least one of Na and K), the reasonwhy ion conductivity increases is assumed to be as follows. Withoutbeing bound by theory, since a radius of the replaced alkali metal ionis larger than a radius of a lithium ion, when some of the Li isreplaced with another alkali metal ion, the solid electrolyte may easilydiffuse Li, compared to the case when a crystal lattice of the solidelectrolyte is only constituted with Li, and thus an ion conduction pathof Li may increase, which may result in increasing conductivity. Also,an amount of the replaced alkali metal M1 may be a value represented bya molar ratio of M1/(Li+M1) (also, referred to as a cation ratio) whichmay be in a range of greater than 0 to about 0.07. When the cation ratiois greater than 0.07, ion conductivity may decrease.

An amount of the alkali metal other than Li is substituted for Li andintroduced into a lattice. However, with increasing the substituting ofthe alkali metal other than Li, the increase of amounts of LiCl and Li₂Swas identified. These phases are not ion conductive materials, thus ionconductivity is assumed to be decreased. Without being bound by theory,this is the reason why the ion conductivity is assumed to be decreasedwhen a cation ratio is greater than about 0.07. For example, when thealkali metal other than Li is Na, an impurity peak is observed at aposition of 30.01°±0.50° 2θ in the X-ray diffraction measurement usingCuKα radiation, and an intensity of the impurity peak is less than apredetermined intensity threshold.

In an embodiment, the solid electrolyte has a composition of Formula 1,and M1 may be Na.(Li_(1-x)M1_(x))_(7-y)PS_(6-y)M2_(y)  Formula 1

wherein 0<x≤0.07 and 0≤y≤2.

According to another embodiment, the solid electrolyte is prepared usinga preparation method including mechanically milling a raw materialmixture including Li₂S, P₂S₅, and M1₂S and LiM₂, or M1M2 at apredetermined ratio to obtain glass; and heat-treating the glass at atemperature equal to or greater than a glass transition temperature ofthe glass to convert the glass into ion conductive glass ceramic. Whenthe glass is first obtained by performing the mechanical milling andthen heat-treating the resultant to convert the glass into glassceramic, a solid electrolyte may be stable and may have desirable ionconductivity.

According to another embodiment, a structure of an all-solid batteryincluding the solid electrolyte will be described. FIG. 1 is across-sectional view of an all-solid battery 1 prepared according to oneaspect of an embodiment. FIG. 1 is illustrated schematically for abetter understanding of the structure of the all-solid battery 1, andthus a ratio of thicknesses of the layers may be different from theillustration in FIG. 1.

As shown in FIG. 1, the all-solid battery 1 according to an embodimentmay include an anode current collector layer 11, an anode layer 9, asolid electrolyte layer 7, a cathode layer 5, and a cathode currentcollector layer 3 that are sequentially stacked from the bottom in thisstated order. The solid electrolyte layer 7 between the anode layer 9and the cathode layer 5 directly contacts the anode layer 9 and thecathode layer 5. The anode layer 9, the solid electrolyte layer 7, andthe cathode layer 5 are each formed of a powder, which may bepress-molded. Also, the all-solid battery 1 may have a flat surface butis not limited thereto and may have, for instance, a shape of a circleor a quadrangle.

As used herein, “all-solid” means that a solvent, i.e., a compoundhaving a vapor pressure of greater than 100 pascals at 20° C., is notincluded.

The anode current collector layer 11 includes a conductor, which may beformed of a metal, for example, copper (Cu), nickel (Ni), stainlesssteel, or nickel-plated steel. For example, a thickness of the anodecurrent collector layer 11 may be in a range of about 10 micrometers(μm) to about 20 μm.

The anode layer 9 may include an anode active material in the form of apowder. For example, an average particle diameter of the anode activematerial may be in a range of about 5 μm to about 20 μm. For example,the amount of the anode active material in the anode layer 9 may be in arange of about 60 weight percent (wt %) to about 95 wt %. The anodelayer 9 may further include a binder, a solid electrolyte material inthe form of powder, or a conducting material that do not generate achemical reaction with the solid electrolyte layer 7.

The anode active material may be any suitable anode active material fora lithium battery available in the art. For example, the anode activematerial may include at least one material selected from lithium metal,a metal that is alloyable with lithium, a transition metal oxide, anon-transition metal oxide, and a carbonaceous material.

Examples of the metal alloyable with lithium are Si, Sn, Al, Ge, Pb, Bi,Sb, a Si—Y′ alloy (where Y′ is an alkali metal, an alkaline earth metal,a Group XIII element, a Group XIV element, a transition metal, a rareearth element, or a combination thereof except for Si), and a Sn—Y′alloy (where Y′ is an alkali metal, an alkali earth metal, a Group XIIIelement, a Group XIV element, a transition metal, a rare earth element,or a combination thereof, except for Sn). In an embodiment, Y′ may beMg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg,Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B,Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, or Te.

Non-limiting examples of the transition metal oxides include, but arenot limited to a lithium titanium oxide, a vanadium oxide, a lithiumvanadium oxide, a niobium oxide, and a combination thereof.

For example, the non-transition metal oxide may be SnO₂ or SiO_(x)(where 0<x<2).

Examples of the carbonaceous material include, but are not limited to,crystalline carbon, amorphous carbon, and mixtures thereof. Examples ofthe crystalline carbon include, but are not limited to, graphite, suchas natural graphite or artificial graphite that is in amorphous, plate,flake, spherical, or fibrous form. Examples of the amorphous carboninclude, but are not limited to, soft carbon (carbon sintered at lowtemperatures), hard carbon, meso-phase pitch carbides, sintered corks,and the like.

The anode active material may be used alone or as a combination of atleast two selected therefrom. A thickness of the anode layer 9 may be ina range of about 50 μm to about 300 μm, but embodiments are not limitedthereto.

The solid electrolyte layer 7 may include the solid electrolytedescribed above. For example, an average particle diameter of the solidelectrolyte may be in a range of about 1 μm to about 10 μm. In anembodiment, a thickness of the solid electrolyte layer 7 may be in arange of about 10 μm to about 200 μm, but embodiments are not limitedthereto.

Without being bound by theory, when the average particle diameter of thesolid electrolyte is within the range of about 1 μm to about 10 μm, thebinding properties increase during a process for forming a solidelectrolyte, which may improve the ion conductivity and lifespancharacteristics of solid electrolyte particles.

When the thickness of the solid electrolyte layer 7 is within the rangeof about 10 μm to about 200 μm, a sufficient migration rate of lithiumions may be secured, and thus a desirable ion conductivity may beachieved.

The cathode layer 5 may include a cathode active material in the form ofpowder. For example, an average particle diameter of the cathode activematerial may be in a range of about 2 μm to about 10 μm. For example, anamount of the cathode active material in the cathode layer 5 may be in arange of about 65 wt % to about 95 wt %. The cathode layer 5 may furtherinclude a binder, a solid electrolyte material, and a conductingmaterial such as carbon nanofibers (CNFs) that do not generate chemicalreactions with the solid electrolyte layer 7. The cathode activematerial may be a material capable of reversibly intercalating anddeintercalating lithium ions. For example, the cathode active materialmay be at least one composite oxide of lithium with a metal selectedfrom among Co, Mn, Ni, and a combination thereof. In some embodiments,the cathode active material may be a compound represented by one of thefollowing formulae: Li_(a)A_(1-b)B¹ _(b)D¹ ₂ (where 0.90≤a≤1.8 and0≤b≤0.5); Li_(a)E_(1-b)B¹ _(b)O_(2-c)D¹ _(c) (where 0.90≤a≤1.8, 0≤b≤0.5,and 0≤c≤0.05); LiE_(2-b)B¹ _(b)O_(4-c)D¹ _(c) (where 0≤b≤0.5 and0≤c≤0.05); Li_(a)Ni_(1-b-c)Co_(b)B¹ _(c)D¹ _(α) (where 0.90≤a≤1.8,0≤b≤0.5, 0≤c≤0.05, and 0<α≤2); Li_(a)Ni_(1-b-c)Co_(b)B¹ _(c)O_(2-α)F¹_(α) (where 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2);Li_(a)Ni_(1-b-c)Co_(b)B¹ _(c)O_(2-α)F¹ ₂ (where 0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, and 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)B¹ _(c)D¹ _(α) (where0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α≤2); Li_(a)Ni_(1-b-c)Mn_(b)B¹_(c)O_(2-α)F¹ _(α) (where 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2);Li_(a)Ni_(1-b-c)Mn_(b)B¹ _(c)O_(2-α)F¹ ₂ (where 0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, and 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂ (where 0.90≤a≤1.8,0≤b≤0.9, 0≤c≤0.5, and 0.001≤d≤0.1); Li_(a)Ni_(b)Co_(c)Mn_(d)GeO₂ (where0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, and 0.001≤e≤0.1); Li_(a)NiG_(b)O₂(where 0.90≤a≤1.8, and 0.001≤b≤0.1); Li_(a)CoG_(b)O₂ (where 0.90≤a≤1.8,and 0.001≤b≤0.1); Li_(a)MnG_(b)O₂ (where 0.90≤a≤1.8, and 0.001≤b≤0.1);Li_(a)Mn₂G_(b)O₄ (where 0.90≤a≤1.8 and 0.001≤b≤0.1); QO₂; QS₂; LiQS₂;V₂O₅; LiV₂O₅; LiI¹O₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃ (0≤f≤2);Li_((3-f))Fe₂(PO₄)₃ (0≤f≤2); and LiFePO₄.

In the formulae above, A may be selected from nickel (Ni), cobalt (Co),manganese (Mn), and combinations thereof; B¹ may be selected fromaluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), chromium (Cr),iron (Fe), magnesium (Mg), strontium (Sr), vanadium (V), a rare earthelement, and combinations thereof; D′ may be selected from oxygen (O),fluorine (F), sulfur (S), phosphorus (P), and combinations thereof; Emay be selected from cobalt (Co), manganese (Mn), and combinationsthereof; F¹ may be selected from fluorine (F), sulfur (S), phosphorus(P), and combinations thereof; G may be selected from aluminum (Al),chromium (Cr), manganese (Mn), iron (Fe), magnesium (Mg), lanthanum(La), cerium (Ce), strontium (Sr), vanadium (V), and combinationsthereof; Q is selected from titanium (Ti), molybdenum (Mo), manganese(Mn), and combinations thereof; I¹ is selected from chromium (Cr),vanadium (V), iron (Fe), scandium (Sc), yttrium (Y), and combinationsthereof; and J may be selected from vanadium (V), chromium (Cr),manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), and combinationsthereof.

For example, the cathode active material may be a lithium cobalt oxide(LCO), a lithium nickel oxide, a lithium nickel cobalt oxide, a lithiumnickel cobalt aluminum (NCA) oxide, a lithium nickel cobalt manganese(NCM) oxide, a lithium manganese oxide, a lithium iron phosphate, anickel sulfide, a copper sulfide, sulfur, an iron oxide, a vanadiumoxide, and a combination thereof. The cathode active material may beused alone or as a combination of at least two selected therefrom. Forexample, a thickness of the cathode layer 5 may be in a range of about50 μm to about 350 μm, but embodiments are not limited thereto.

The cathode current collector layer 3 is formed of a conductor, forexample, a metal such as aluminum (Al) or stainless steel. For example,a thickness of the cathode current collector 3 may be in a range ofabout 10 μm to about 20 μm, but embodiments are not limited thereto.

Example 1

First, reagents of Li₂S, Na₂S, P₂S₅, and LiCl were measured to form thedesired composition (Li_(5.635)Na_(0.115))PS_(4.75)Cl_(1.25), and amechanical milling treatment was performed thereon to mix the reagentsin a planetary ball mill for 20 hours. The mechanical milling treatmentwas performed at a rotation rate of 380 rpm, at room temperature (about25° C.), and in an argon atmosphere.

300 mg of a powder material having a composition of(Li_(5.635)Na_(0.115))PS_(4.75)Cl_(1.25) (a cation ratio=0.02) asobtained from the mechanical milling treatment was pressed (at apressure of about 400 mega pascals per square centimeter (MPa/cm²)) toobtain a pellet having a diameter of about 13 mm and a thickness ofabout 0.8 mm. The pellet thus obtained was coated with a gold film andplaced into a carbon furnace, and the carbon furnace was vacuum-sealedby using a quartz glass tube. A temperature of the vacuum-sealed pelletwas increased from room temperature to 550° C. using an electric furnaceat a rate of 1.0° C./min, heat-treated at 550° C. for 6 hours, and thencooled to room temperature at a rate of 1.0° C./min to obtain a sample(a solid electrolyte) of Example 1.

The sample was pulverized by using an agate mortar, X-ray crystaldiffraction using CuKα radiation was performed thereon, and formation ofthe desired crystals having an Argyrodite structure was confirmed (FIG.2). Also, in FIG. 2, peak A was a peak at 15.42°±0.50° 2θ, peak B was apeak at 17.87°±0.50° 2θ, peak C was a peak at 25.48°±0.50° 2θ, peak Dwas a peak at 30.01°±0.50° 2θ, and peak E was a peak at 31.38°±0.50° 2θ.

Also, a base line was removed by using the X-ray diffraction patternmeasured by this method. When the maximum intensity of the peak D at30.01°±0.50° 2θ was referred to as IA and the maximum intensity of thepeak F at 33.65°±0.50° 2θ was referred to as IB, IB/IA was 0.013.

The ion conductivity of the sample was measured as follows. The samplewas pulverized by using an agate mortar and was pressed (at a pressureof 400 MPa/cm²) to prepare a pellet. Also, an In foil (at a thickness of50 μm) was provided on both surfaces of the pellet to prepare a pelletfor measuring ion conductivity. The pellet for measuring ionconductivity exhibited an ion conductivity of 4.5×10⁻³ siemens percentimeter (S/cm) at room temperature.

Example 2

A sample of Example 2 was prepared the same as in Example 1, except thatan amount of Na and the corresponding amount of Li added to the samplewere different from those used in Example 1, but a material composition,a process, and a measurement method were the same as those used inExample 1.

In Example 2, reagents of Li₂S, Na₂S, P₂S₅, and LiCl were measured toform the desired composition (Li_(5.4625)Na_(0.2875))PS_(4.75)Cl_(1.25),and a mechanical milling treatment and heat-treatment were performedthereon to prepare a sample (a solid electrolyte) of Example 2 having acomposition of (Li_(5.4625)Na_(0.2875))PS_(4.75)Cl_(1.25) (a cationratio=0.05).

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, in FIG. 2, IB/IA was 0.083. Anion conductivity of the sample was measured at room temperature, and theion conductivity of the sample was confirmed to be 3.1×10⁻³ S/cm.

Example 3

A sample of Example 3 was prepared the same as in Example 1, except thatthe amount of Na and the corresponding amount of Li added to the samplewere different from those used in Example 1, but a material composition,a process, and a measurement method were the same as those used inExample 1.

In Example 3, reagents of Li₂S, Na₂S, P₂S₅, and LiCl were measured toform the desired composition (Li_(5.6925)Na_(0.0575))PS_(4.75)Cl_(1.25),and a mechanical milling treatment and heat-treatment were performedthereon to prepare a sample (a solid electrolyte) of Example 3 having acomposition of (Li_(5.6925)Na_(0.0575))PS_(4.75)Cl_(1.25) (a cationratio=0.01).

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, in FIG. 2, IB/IA was 0.0184. Anion conductivity of the sample was measured at room temperature, and theion conductivity of the sample was confirmed to be 5.1×10⁻³ S/cm.

Example 4

A sample of Example 4 was prepared the same as in Example 1, except thatthe amount of Na and the corresponding amount of Li added to the samplewere different from those used in Example 1, but a material composition,a process, and a measurement method were the same as those used inExample 1.

In Example 4, reagents of Li₂S, Na₂S, P₂S₅, and LiCl were measured toform the desired composition (Li_(5.5775)Na_(0.1725))PS_(4.75)Cl_(1.25),and a mechanical milling treatment and heat-treatment were performedthereon to prepare a sample (a solid electrolyte) of Example 4 having acomposition of (Li_(5.5775)Na_(0.1725))PS_(4.75)Cl_(1.25) (a cationratio=0.03).

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, in FIG. 2, IB/IA was 0.0481. Anion conductivity of the sample was measured at room temperature, and theion conductivity of the sample was confirmed to be 2.8×10⁻³ S/cm.

Example 5

The sample obtained in Example 3 was pulverized by using an agate mortarand pressed (at a pressure of 400 MPa/cm²) to prepare a pellet. Also, aLi foil and an In foil were provided on the pellet at thicknesses of 100μm and 500 μm, respectively, to prepare a pellet for a cyclicvoltammetry (CV) measurement. The results of the CV measurement areshown in FIG. 7A.

Comparative Example 1

A sample of Comparative Example 1 was prepared the same as in Example 1,except that the sample of Comparative Example 1 only included Li and didnot include Na, but a material composition, a process, and a measurementmethod were the same as those used in Example 1.

In Comparative Example 1, reagents of Li₂S, P₂S₅, and LiCl were measuredto form the desired composition Li_(5.75)PS_(4.75)Cl_(1.25), and asample (a solid electrolyte) of Comparative Example 1 having acomposition of Li_(5.75)PS_(4.75)Cl_(1.25) was obtained in the samemanner as in Example 1.

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, IB/IA was 0.0. An ionconductivity of the sample was measured at room temperature, and the ionconductivity of the sample was confirmed to be 2.3×10⁻³ S/cm.

Comparative Example 2

A sample of Comparative Example 2 was prepared the same as in Example 1,except that the amount of Na and the corresponding amount of Li added tothe sample were different from those used in Example 1, but a materialcomposition, a process, and a measurement method were the same as thoseused in Example 1.

In Comparative Example 2, reagents of Li₂S, Na₂S, P₂S₅, and LiCl weremeasured to form the desired composition(Li_(5.175)Na_(0.575))PS_(4.75)Cl_(1.25), and a mechanical millingtreatment and heat-treatment were performed thereon to prepare a sample(a solid electrolyte) of Comparative Example 2 having a composition of(Li_(5.175)Na_(0.575))PS_(4.75)Cl_(1.25) (a cation ratio=0.10).

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, IB/IA was 0.182. An ionconductivity of the sample was measured at room temperature, and the ionconductivity of the sample was confirmed to be 1.7×10⁻⁴ S/cm.

Comparative Example 3

A sample of Comparative Example 3 was prepared the same as in Example 1,except that the amount of Na and the corresponding amount of Li added tothe sample were different from those used in Example 1, but a materialcomposition, a process, and a measurement method were the same as thoseused in Example 1.

In Comparative Example 3, reagents of Li₂S, Na₂S, P₂S₅, and LiCl weremeasured to form the desired composition(Li_(5.319)Na_(0.431))PS_(4.75)Cl_(1.25), and a mechanical millingtreatment and heat-treatment were performed thereon to prepare a sample(a solid electrolyte) of Comparative Example 3 having a composition of(Li_(5.319)Na_(0.431))PS_(4.75)Cl_(1.25) (a cation ratio=0.075).

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, IB/IA was 0.201. An ionconductivity of the sample was measured at room temperature, and the ionconductivity of the sample was confirmed to be 9.7×10⁻⁴ S/cm.

Comparative Example 4

A sample of Comparative Example 4 was prepared the same as in Example 1,except that the amount of Na and the corresponding amount of Li added tothe sample were different from those used in Example 1, but a materialcomposition, a process, and a measurement method were the same as thoseused in Example 1.

In Comparative Example 4, reagents of Li₂S, Na₂S, P₂S₅, and LiCl weremeasured to form the desired composition(Li_(4.888)Na_(0.863))PS_(4.75)Cl_(1.25), and a mechanical millingtreatment and heat-treatment were performed thereon to prepare a sample(a solid electrolyte) of Comparative Example 4 having a composition of(Li_(4.888)Na_(0.863))PS_(4.75)Cl_(1.25) (a cation ratio=0.15).

X-ray crystal diffraction using CuKα radiation was performed on thesample, and formation of the desired crystals having an Argyroditestructure was confirmed (FIG. 2). Also, IB/IA was 0.359. An ionconductivity of the sample was measured at room temperature, and the ionconductivity of the sample was confirmed to be 3.3×10⁻⁶ S/cm.

Comparative Example 5

A commercially available sample (Li₁₀SnP₂S₁₂) was pulverized by using anagate mortar and pressed (at a pressure of 400 MPa/cm²) to prepare apellet. Also, a Li foil and an In foil were provided on the pellet atthicknesses of 100 μm and 500 μm, respectively, to prepare a pellet fora CV measurement. The results of the CV measurement are shown in FIG.7B.

Evaluation 1

Compared to the sample of Comparative Example 1, which included Li as analkali metal but did not include Na, the samples of Examples 1 to 4including Na and having a cation ratio in a range of about 0.01 to about0.05 had an ion conductivity that increased about 1.4 fold to 2.2 fold.Ion conductivities of the samples prepared in Comparative Examples 2 to4 having a cation ratio in a range of about 0.075 to about 0.15increased only about 0.0015 fold to about 0.42 fold from that of thesample prepared in Comparative Example 1, and it is deemed that this wasbecause the added Na was not all introduced to the Argyrodite structurebut some of Na became impurities that suppressed improving ionconductivity.

An ion conductivity of the sample prepared in Comparative Example 3having a cation ratio of 0.075 was 1.7×10⁻⁴ S/cm, and this was 0.42 foldof that of the sample prepared in Comparative Example 1. The cationratio at which the ion conductivity improving effect may be exhibited byaddition of Na is deemed to be less than 0.075.

Evaluation 2

In Example 5, currents associated with Li dissolution and Li extractionwere observed by the C-V (Capacitance-voltage) measurement as a reactionbetween the solid electrolyte and a Li metal occurred, but, inComparative Example 5, a Li dissolution current was not observed butonly a current indicating Li extraction was observed. This is deemed tobe because the solid electrolyte and the Li metal pre-reacted on aninterface in the case of Comparative Example 5, which made Li extractioninsufficient during the CV measurement, and thus the Li dissolutionreaction almost did not occur.

Example 6

In Example 6, a test battery was prepared by using the solid electrolyteof Example 2, and battery characteristics of the test battery weremeasured.

LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ (NCA) as a cathode active material, thesolid electrolyte of Example 2, and CNFs as a conducting material weremixed at a ratio of the cathode active material:solidelectrolyte:CNFs=83:15:2 wt % to prepare a cathode mixture. Also, ametal lithium foil was used as an anode.

The cathode mixture, the solid electrolyte, and an anode mixture werestacked at amounts of 10 mg, 150 mg, 2 mg, respectively, and pressed ata pressure of 4 tons per square centimeter (ton/cm²) to prepare a testbattery. In the test battery, a solid electrolyte was not decomposed bythe anode and was stable.

The test battery thus prepared was charged at 25° C. with a constantcurrent of 0.05 C until a voltage of an upper limit was 4.0 volts (V)and discharged with a constant current of 0.05 C until a voltage of alower limit was 2.5 V to measure an initial battery capacity. Then, thebattery was discharged with a current of 0.05 C (FIG. 3), 0.5 C (FIG.4), or 1 C (FIG. 5) to measure rate characteristics with respect toother batteries. The characteristics of the example according to anembodiment of the present disclosure are shown as G in FIGS. 3 to 5.

Also, while the test battery was charged to a voltage of an upper limit4.0 V, an impedance of the battery was measured to evaluate a batteryinternal resistance. An internal resistance of the battery of theexample according to an embodiment is shown as G in FIG. 6.

Comparative Example 6

In Comparative Example 6, a test battery was prepared by using the solidelectrolyte of Comparative Example 1, and battery characteristics of thetest battery were measured. A preparation method of the test battery anda measurement method of the battery characteristics were the same asthose used in Example 6. Characteristics and internal resistance of thebattery prepared in Comparative Example 6 were shown as H in FIGS. 3 to6.

Evaluation 3

In FIGS. 3 to 5, it may be known that a discharge curve of the batteryprepared in Example 6 had a high voltage and a high electric capacity inall 0.05 C, 0.5 C, and 1 C. Particularly, when a discharge currentincreased, the batteries had a high voltage and a high electriccapacity, compared to those of the battery prepared in Example 6.

In FIG. 6, a graph shape of the internal resistance was the same, but Z′of Example 6 was smaller than that of Comparative Example 5, and thus itmay be known that a resistance of the solid electrolyte between thecathode and the anode was small.

Other Examples

Examples described above are provided herein as examples, and thepresent disclosure is not limited thereto.

M1 is not limited to Na. That is, M1 may be K instead of Na or acombination of a Na and K.

As a raw material used in preparation of the solid electrolyte, a sodiumhalide or a potassium halide may be used as precursors to a lithiumhalide.

M2 is not limited to Cl and one selected from Br, F, and I or anymaterial including Cl may be used. Any of Br, F, and I may be used.

As described above, according to one or more embodiments, the solidelectrolyte has a trace amount of at least one selected from Na, K, anda combination thereof with respect to Li, and thus is stable withrespect to a metal lithium and has a desirably high ion conductivity atthe same time.

It should be understood that embodiments described herein should beconsidered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in other embodiments.

While one or more embodiments have been described with reference to thefigures, it will be understood by those of ordinary skill in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope as defined by the following claims.

What is claimed is:
 1. A solid electrolyte for an all-solid secondarybattery, the solid electrolyte comprising a compound of Formula 1:(Li_(1-x)M1_(x))_(7-y)PS_(6-y)M2_(y)  Formula 1 wherein, in Formula 1,M1 is Na, M2 is Cl, and molar amounts of Li and M1 satisfy0.01≤M1/(Li+M1)≤0.07, and x and y satisfy 0<x≤0.07 and 0<y≤2, andwherein the solid electrolyte has peaks at positions of 15.42°±0.50° 2θ,17.87°±0.50° 2θ, 25.48°±0.50° 2θ, 30.01°±0.50° 2θ, and 31.38°±0.50° 2θwhen analyzed by X-ray diffraction using CuKα radiation.
 2. The solidelectrolyte of claim 1, wherein the solid electrolyte has an Argyroditestructure.
 3. The solid electrolyte of claim 1, wherein the solidelectrolyte further comprises a first peak at a position of 30.01°±0.50°2θ and at a second peak at a position of 33.65°±0.50° 2θ when analyzedby X-ray diffraction using CuKα radiation, and wherein a ratio of anintensity of the second peak to an intensity of the first peak is lessthan about 0.1.
 4. The solid electrolyte of claim 1 comprising(Li_(5.6925)Na_(0.0575))PS_(4.75)Cl_(1.25).
 5. A secondary batterycomprising: a cathode comprising a cathode active material; an anodecomprising an anode active material; and a solid electrolyte layercomprising the solid electrolyte of claim
 1. 6. The secondary battery ofclaim 5, wherein the cathode active material comprises at least one of alithium cobalt oxide, a lithium nickel oxide, a lithium nickel cobaltoxide, a lithium nickel cobalt aluminum oxide, a lithium nickel cobaltmanganese oxide, a lithium manganese oxide, a lithium iron phosphate, anickel sulfide, a copper sulfide, sulfur, an iron oxide, and a vanadiumoxide.
 7. The secondary battery of claim 6, wherein the cathode activematerial further comprises the solid electrolyte of claim 1 and carbonnanofibers.
 8. The secondary battery of claim 5, wherein the anodeactive material comprises at least one of a carbon active material, ametal active material, an oxide active material.
 9. The secondarybattery of claim 5, wherein an average particle diameter of the solidelectrolyte layer is in a range of about 1 micrometer to about 10micrometers.
 10. The secondary battery of claim 5, wherein a thicknessof the solid electrolyte layer is in a range of about 10 micrometers toabout 200 micrometers.
 11. The secondary battery of claim 5, wherein thesecondary battery is an all-solid secondary battery.
 12. A method ofpreparing the solid electrolyte of claim 1, the method comprising:mechanically milling a mixture comprising Li₂S, P₂S₅, and M1₂S and LiM2,or M1M2 to obtain a glass; and heat-treating the glass at a glasstransition temperature or greater to convert the glass into and ionconductive glass ceramic and obtain the solid electrolyte.
 13. The solidelectrolyte of claim 1, wherein the solid electrolyte has an ionconductivity of greater than 2×10⁻³ Siemens per centimeter.
 14. Asecondary battery comprising: a cathode comprising a cathode activematerial able to reversibly intercalate lithium ions; an anodecomprising an anode active material comprising lithium; and a solidelectrolyte layer comprising a solid electrolyte of the formula(Li_(1-x)Na_(x))_(7-y)PS_(6-y)Cl_(y), wherein molar amounts of Li and Nasatisfy 0.01≤M1/(Li+Na)≤0.07, and x and y satisfy 0<x≤0.07 and 0<y≤2.